Fill a medium saucepan or deep sauté pan with water halfway up the sides and bring to a boil. Once boiling, add 1 teaspoon of salt to the water.
While the water is coming to a boil, prepare an ice bath in a large bowl and set aside.
Trim the asparagus by snapping off the tough bottom of each stalk where it naturally breaks when bent. Add the asparagus to the boiling water and cook for 3 to 5 minutes, or until the asparagus is tender enough to be speared through with a fork. Drain the water and plunge the hot asparagus into the ice bath for 30 seconds. Remove the asparagus and drain on a clean kitchen towel.
While the asparagus is cooking, whisk together the vinegar and mustard in a small bowl, then add the remaining 1/4 teaspoon salt and pepper and allow the salt to dissolve. Pour the olive oil in a thin stream into the mustard mixture, whisking constantly until everything is fully emulsified. Fold the crumbled feta into the dressing with a rubber spatula and set aside. If the dressing separates a little, stir it together again before serving.
Place the asparagus in a serving bowl or plate and evenly drizzle with the vinaigrette; toss lightly with a spoon or fork to make sure all of the asparagus is coated. Serve immediately.
Make-Ahead Tips: The asparagus can be blanched a few hours in advance. The feta vinaigrette can also be made up to a day ahead of time. When you are ready to serve the dish, simply bring the asparagus to room temperature and bring everything together as outlined in Step 5.
Reprinted with permission from The Naptime Chef by Kelsey Banfield, Running Press, 2012.